A drive device with a safety device for special operation is described in German Patents No. 43 30 823. In this patent a redundant safety device is provided to monitor the motor, verifying that the motor speed does not exceed a predetermined maximum value in special operation and to interrupt the power supply to the motor when the rotational speed is greater than the predetermined maximum value. To determine the rotational speed, two different signals are detected so that the first signal is obtained from a rotational speed sensor and the second signal is derived in at least one phase feeder to the motor from the variation of current over time detected for this purpose by a second sensor. If the detected rotational speed exceeds a predetermined maximum value, the energy supply to the motor is interrupted by switching a circuit breaker upstream from a power rectifier and in addition switching off the power inverter.
Additional monitoring of the actuation signals for the interruption of the power supply is however not provided. A break in a feeder or a short-circuit is not monitored separately.